1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a reflective liquid crystal display device having a cholesteric color filter layer.
2. Discussion of the Related Art
A liquid crystal display (LCD) device has been in the spotlight as a next generation display device having high value added because of its low power consumption and good portability.
An active matrix liquid crystal display (AMLCD) device, which includes thin film transistors as a switching device for a plurality of pixels, has been widely used due to its high resolution and fast moving images.
Because the LCD device is not luminescent, it needs an additional light source in order to display images. In general, the LCD device has a backlight behind a liquid crystal panel as a light source, and such a LCD is usually referred to as a transmissive LCD device. In the transmissive type, light incident from the backlight penetrates the liquid crystal panel, and the amount of the transmitted light is controlled according to the alignment of liquid crystal molecules. Because the transmissive LCD device uses the backlight as a light source, it can display a bright image in dark surroundings. However, the amount of the transmitted light is very small for the amount of light incident from the backlight. That is, because only 7% of the light incident from the backlight is transmitted through the liquid crystal panel, the brightness of the backlight should be increased in order to increase the brightness of the LCD device. Consequently, the transmissive LCD device has high power consumption due to the backlight.
To solve the problems in the transmissive LCD device, a reflective LCD device has been proposed. In the reflective LCD device, sunlight or artificial light is used as a light source of the LCD device. The light incident from the outside is reflected at a reflective plate of the LCD device according to the arrangement of the liquid crystal molecules. Since there is no backlight, the reflective LCD device has much lower power consumption than the transmissive LCD device. By the way, the reflective LCD device, generally, includes an absorptive color filter layer, which is made of pigments or dyes, the same as the transmissive LCD device. The reflective LCD device also has a disadvantage of low light transmittance due to the absorptive color filter layer.
To improve the light transmittance in the reflective LCD device, a cholesteric liquid crystal (CLC) color filter has been researched and developed. As the CLC color filter selectively reflects and transmits light, the CLC color filter can emit light of high purity in color. Additionally, the CLC color filter functions both as a color filter layer and as a reflector. Therefore, since the reflective LCD device including the CLC color filter does not require an additional reflector, manufacturing processes are decreased and image quality is improved.
Liquid crystal molecules of the CLC are arranged in a helical structure. The helical structure is characterized by a helical direction and a pitch, which is a cycle of the helical structure. A color tone of light reflected by the CLC depends on the pitch. That is, an average wavelength of the reflected light is the pitch times an average reflective index of the CLC, and is represented by the following formula.λ=n(avg)·pitch,wherein the n(avg) is the average reflective index of the CLC.
For example, if the CLC has the average reflective index of about 1.5 and the pitch of the CLC is about 430 nm, the light reflected by the CLC has the average wavelength of about 650 nm, and is reddish. Greenish or bluish light may be reflected by changing the pitch of the CLC.
A reflective LCD device including a CLC color filter will be explained in detail with reference to the following figures.
FIG. 1 is a plan view of a reflective LCD device including a cholesteric liquid crystal (CLC) color filter according to the related art. As shown in the figure, a gate line 22 of a horizontal direction and a data line 32 of a vertical direction in the context of the figure cross each other to define a pixel area. A thin film transistor, which includes a gate electrode 24, a source electrode 34 and a drain electrode 36, is formed at the crossing of the gate line 22 and the data line 32 to function as a switching element. The gate electrode 24 is connected to the gate line 22; the source electrode 34 is connected to the data line 32; and the drain electrode 36 is spaced apart from the source electrode 34. The thin film transistor further includes an active layer 28, and the active layer 28 between the source electrode 34 and the drain electrode 36 becomes a channel of the thin film transistor. A passivation layer 40 is formed over the thin film transistor.
A capacitor electrode 37 overlaps the gate line 22 to form a storage capacitor. The capacitor electrode 37 may be made of the same material as the data line 32.
In the pixel area, a pixel electrode 42 is formed. The pixel electrode 42 is connected to the drain electrode 36 through a drain contact hole 40a through the passivation layer 40 and is connected to the capacitor electrode 37 through a capacitor contact hole 40b through the passivation layer 40. The pixel electrode 42 overlaps the data line 32.
To prevent light leakage in a region except for the pixel area, a black matrix 38 is formed corresponding to edges of the pixel electrode 42. The black matrix 38 also covers a part of the gate line 22, the data line 32 and the channel of the thin film transistor.
Although not shown in the figure, a CLC color filter layer is formed, and the CLC color filter layer reflects light of wavelengths corresponding to one of red, green and blue colors by pixel areas.
FIGS. 2A and 2B are cross sectional views along the line IIA—IIA and the line IIB—IIB of FIG. 1, respectively.
In FIGS. 2A and 2B, a first substrate 10 and a second substrate 50 are spaced apart and facing each other. The first substrate 10 may be made of a transparent substrate. A gate electrode 24 is formed on an inner surface of the first substrate 10, and a gate insulating layer 26 covers the gate electrode 24. An active layer 28 is formed on the gate insulating layer 26 over the gate electrode 24, and a source electrode 34 and a drain electrode 36 are formed on the active layer 28. As state above, the gate electrode 24, the active layer 28, the source electrode 34 and the drain electrode 36 form a thin film transistor, and the active layer 28 exposed between the source electrode 34 and the drain electrode 36 becomes a channel of a thin film transistor. Additionally, a data line 32, which is made of the same material as the source electrode 34 and the drain electrode 36, is formed on the gate insulating layer 26.
A black matrix 38 is formed on the data line 32, the source electrode 34 and the drain electrode 36, and covers the data line 32 and the channel of the thin film transistor. The black matrix 38 may be made of a black resin. The black matrix 38 blocks light leakage in edge portions of a pixel area, that is, close by the data line 32, and prevents light from reaching the channel of the thin film transistor.
A passivation layer 40 is formed on the black matrix 38, and the passivation layer 40 is made of an organic material having a relatively low dielectric constant. The passivation layer 40 includes a drain contact hole 40a exposing the drain electrode 36.
A pixel electrode 42 is formed on the passivation layer 40, and the pixel electrode 42 is connected to the drain electrode 36 through the drain contact hole 40a. The pixel electrode 42 may overlap the data line 32 to increase an aperture ratio of the LCD device.
On the other hand, an absorption layer 52 is formed on an inner surface of the second substrate 50, and a CLC color filter layer, which includes sub color filters 54a and 54b, is formed on the absorption layer 52. Each sub color filter of the CLC color filter layer 54a and 54b corresponds to one pixel area and reflects light of wavelengths corresponding to one of red, green and blue colors. A common electrode 56 is formed on the CLC color filter layer 54a and 54b. The common electrode 56 is made of a transparent conducting material.
A liquid crystal layer 60 is disposed between the common electrode 56 and the pixel electrode 42.
A retardation film 72 and a polarizer 74 are subsequently arranged on an outer surface of the first substrate 10. The retardation film 72 may be a quarter wave plate (QWP) having a retardation of λ/4, and the polarizer 74 may be a linear polarizer that transmits only linearly polarized light parallel to its transmission axis.
Like this, the black matrix is formed to prevent photocurrent from being generated in the thin film transistor due to light and to block light leakage nearby the data line. However, since the black matrix of the related art is made of a resin that has relatively a low resistivity and a high dielectric constant, leakage current may be generated in the thin film transistor.